Cathodes have been prepared for many years using the alkaline earth oxides of appropriate crystal structures to give electron emission when heated to 700.degree.-1100.degree. C. The oxides are very sensitive to cathode poisoning which is believed to close off the fine porous structure that aids electron emission. For optimum emission, the industry uses controlled composition mixed CaSrBa carbonates, (known as triple carbonate). The carbonate breaks down during the initial heating of the cathode in vacuum during the evacuation and seal-off of the cathode ray tube (CRT) or the vacuum tube. The formation of the oxide sometimes termed thermal activation) from the carbonate, is performed while the excess Co.sub.2 is being pumped away. Illustratively, the oxides are sensitive to water vapor which damages the triple oxide crystal structures. Hence, the industry prefers carbonate deposition plus later decomposition to the oxides. Normally, the carbonates for both electron tubes and cathode ray tubes are sprayed or dipcoated onto the cathode structure, and an organic binder is used to make the carbonate particles stick to the cathode metal substrate. These binders are often based on polymethacrylate or nitrocellulose and are chosen because they leave the carbonate crystal morphology intact after they are "burned-off", during the initial heating of the thermal activation step.
A background reference concerning the foregoing is: Handbook of Materials and Techniques for Vacuum Devices, by W. H. Kohl, Reinhold Publishing Corp., 1967.
Very few instances of patterned cathodes have been reported in the background literature. Generally, the design of devices did not require it. Some work has been reported on a photoresist process for patterning triple carbonates. These are various Stanford Research Institute Quarterly Reports on Low Temperature Thermionic Emitter prepared for the NASA on contract 12-607, particularly the Quarterly Report No. 3 dated Nov. 15, 1968, by D. U. Geppert et al which describes cathode coatings on coplanar diode vacuum devices. The method reported in the Quarterly Report, No. 3 provides accurate patterning of triple carbonates. The method uses a negative thin film resist Carbonate particles are loaded into the photoresist (up to 75% solids content) and milled in a ball mill to produce a slurry of appropriate viscosity to be spin coated on a cathode. Typically, 70% CaSrBa carbonate crystals are incorporated into Kodak photoresist and exposed to conventional photo sources. To avoid poor resolution, the mixtures are heavily ball milled. The typical image sizes of fractions of a mil are not adequate for practice of this invention. The Stanford Research Institute loaded photoresist method has several difficulties which restrict the application of the technology to finer limits. Practice of this method has the following attendant problems: (a) difficulty in locating wafer registration marks through opaque resist; (b) poor resolution because of light scattering of solids; (c) difficulty of spinning loaded resists; (d) solid residues left on the developed areas need an undercoat of pure resist to avoid extraneous emission therefrom; (e) there is a layer or residue of the resist layer between emission layer and metal of cathode; (f) as the triple carbonate is in the surface of the resist, it is exposed to comtamination e.g., by gasses, causing subsequent loss of emission; (g) and the heavy ball milling of the carbonate particles necessary to reduce the crystal size in order to achieve improved resolution, adversely affects the emission efficiency (large carbonate crystals are used to give open oxide structures).
Triple carbonates of appropriate crystal size can be bought commercially. Background reference is in chapter 16, of the notebook by W. H. Kohl entitled "Cathodes and Heaters". Filamentary cathode emitters which were coated with alkaline-earth carbonates by cataphoresis (cathode deposition by electrophoresis) are described on page 514 of the noted book by Kohl.
Further, delineation of cataphoretically deposited filamentary cathodes by local electrolytic removal of the carbonate was known heretofore. It has been known that: needle crystals of carbonate typically 5-15 microns long and up to a micron in cross-section are particularly good crystal structures for obtaining electron emission and that the needle crystals can be deposited cataphoretically virtually normal with respect to the filamentary surface upon which they were being coated.
The prior art practice has provided microminiature planar cathode-grid structures for vacuum tubes in which the principles of this invention can be beneficially applied. Illustrative of such structures are those disclosed in U.S. Pat. No. 4,138,622 filed Aug. 4, 1977 by J. B. McCormick et al. and issued Feb. 6, 1977 for High Temperature Electronic Gain Device; and U.S. patent application Ser. No. 148,899 filed May 2, 1980 by S. W. Depp and B. Piggin and commonly assigned for Multiple Electron Beam Cathode Ray Tube. The disclosure of this patent application by Depp et al is incorporated herein by reference.
The devices referred to in the aforesaid patent and patent application require triple carbonate patterns precisely and clearly delineated since the grid is virtually in the same plane and on the same substrate surface as the cathode. Any spurious imprecise pattern of carbonate deposition risks anomalous uncontrolled grid emission since the grid is at the same temperature 700.degree.-1100.degree. C. as the cathode metal. The ultimate technological limit of microminiaturization to which these devices can be pushed depends on the critical patterning resolution of the carbonate particles.